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Establishment of a value creation chain with European SMEs for the efficient production of diffractive structured glass optics
– Next Generation Optics –

Final Report Summary - NGOPTICS (Establishment of a value creation chain with European SMEs for the efficient production of diffractive structured glass optics – Next Generation Optics –)

Executive Summary:
Miniaturization, function integration and higher imaging quality can be seen as the current trend in the manufacturing and development of high precision and high performance glass optics. In this context, diffractive optical elements depict an innovative way for the improvement of optical systems by adding a defined structure on top of the refractive surface. In general diffractive elements can be separated into two different types, binary and blazed structures (Figure 1).
The technology of precision glass moulding is the enabling technology for the high volume production of precise glass optics with complex geometries. In order to enable the precision glass moulding for manufacturing of blazed diffractive optics the manufacturing of the moulds was the key challenge and the main goal of the project “Next Generation Optics – NGOptics”. Binary structures were not observed due to the limitation to perfectly planar surfaces and the limit of the height of the structures.

In case of blazed diffractive elements the moulds can be machined by a cutting process with defined cutting edge like diamond turning or by a process with undefined cutting edge such as grinding. Both processes provided “half radius tools”. As half radius diamond tools are state of the art, a process for dressing of ultra-fine grained diamond grinding wheels to a minimum radius was developed. For the diffractive structures defined in the project the process of grinding at the end was not suitable due to the very small structure size. Therefore, the process of diamond turning with a tool radius down to 2 µm was used.

In the project two different demonstrators were designed by the SMEs. The correction of the chromatic error when light of different wavelengths passes a curved surface was used by POG for their design of an IR laser telescope for a wavelength of 1543 nm. The main goal was to reduce the number of lenses in the system. The second possibility to use diffractive elements is to use the converse effect; this is the separation of the light into different wavelength. This is used by FocalSpec by developing a new film thickness measurement system. The light beam is separated into the different colours and each colour is focused on the surface of the object at different heights. By detecting the colour, which is in focus on the surface, the distance between the sensor and the object can be measured very accurately. In case of a transparent film the thickness can be measured by the difference between the reflections of the different colours. Currently a system based on binary diffractive elements is used. A new off-axis diffractive optic increases the measurement quality, the efficiency of the measurement technique and decreases the cost for manufacturing significantly. These lenses were requested by FocalSpec and were defined as the second demonstrator. Both demonstrators were successfully moulded by precision glass moulding in the project.

Project Context and Objectives:
In the market of manufacturing and development of high performance glass optics, the miniaturization, function integration and higher imaging quality can be seen as the current trend and the main target of any company involved. In general diffractive optical elements depict an innovative way for the improvement of optical systems by increasing the optical quality or the optical functionality by adding a defined structure on top of the refractive surface. In general diffractive structures can be separated into two different types, binary and blazed. Binary structures can be generated by laser beam for example and offer an insufficient efficiency for most photonics applications due to the limitation to perfectly planar surfaces and the limit of the height of the structures. In contrast blazed gratings can be generated by cutting processes like grinding or diamond turning.

Next to the high complexity of the glass optics, a high volume production technique is needed and can be found in the technologyof Precision glass moulding. In order to enable precision glass moulding for the manufacturing of blazed diffractive optics the mould manufacturing and the development of a stable glass moulding process were the key challenges and the main goals of the project. Within the NGOptics project, the following developments and objectives towards the realization of diffractive glass optics were achieved:

- Selection of relevant materials and coatings for precision glass moulding
- Manufacturing of moulds for precision glass moulding by diamond turning and ultra-sonic assisted diamond turning in relevant materials
- Dressing of ultra-fine grained diamond grinding wheels for generating diffractive structures in brittle hard materials
- Development of a stable precision glass moulding process for the manufacturing of diffractive glass optics

Project Results:
The specifications of the NGOptics demonstrators were defined at the beginning of the project by the SMEs. For this, all partners delivered detailed input from their area of expertise. VTT performed a study on the classification of diffractive elements. Detailed information regarding the minimal dimensions and geometry of diamond tools and grinding wheels were contributed by Contour Finetooling and Technodiamant. Fraunhofer IPT and Aixtooling are providing the knowledge about mould manufacturing by grinding or diamond turning and precision glass moulding of glass elements.

Diffractive optical elements depict an innovative method for the improvement of optical systems towards the trend of miniaturization, function integration and higher imaging quality. Due to their different physical principal compared to refractive optics, they provide a higher-degree of freedom for optic designers. By adding defined structure with step heights in the wavelength dimension onto the refractive surface, further optical functionalities can be added to the optical element itself. Thus, the same optical function can be realized using less space. This leads to a reduction of weight and much easier alignment for example. Furthermore diffractive structures can be used for correcting the chromatic error occurring when light with different wavelength passes a curved surface. The diffractive effect is contrary to the dispersion of the light and can therewith be corrected with a diffractive structure. In general diffractive structures can be classified into two groups: binary and blazed. In this project only blazed structure were studied.

POG designed a new optical system for a revision of an existing IR laser telescope for a wavelength of 1543 nm with the main goals of reducing the number of used lenses and higher imaging quality. The direct use of diffraction in order to separate the wavelengths is used by FocalSpec. They have developed a new film thickness measurement system. The light beam is separated into the different colours, each colour being focused on the surface of the object at different heights. By detecting which colour is in focus on the surface of the object by a receiving optical system the distance between the sensor and the object can be measured very accurately.

Based on the specifications of the demonstrator lenses, a standardized mould design for single and multi-cavity concepts was designed to mould the demonstrators and in order to be able to react efficiently towards future requests. Both concepts have several advantages and disadvantages for the process of glass moulding and mould manufacturing:
Single cavity:
- Advantages:
Smaller area to be machined on the mould inserts
One insert necessary for aspheric side
- Disadvantages:
Intermitted cut, more important the harder the material is
Multi cavity:
Advantages:
High outputing moulding process
Continuous cut while mould manufacturing
Disadvantages:
Larger diffractive area to be machined
More complex tooling to get alignment to asphere on opposite side

Diffractive glass optics cannot be manufactured in the conventional way by grinding and polishing. The process of precision glass moulding is the enabling technologyfor the high volume production of precise glass optics with complex geometries. In order to enable the precision glass moulding of diffractive optics the mould manufacturing is the key factor. Based on the mould material two different processes can be used: ultra-precision grinding and diamond turning. For both processes special tools are needed. For the machining of the micro structured moulds in hard and brittle materials ultra-fine grained diamond grinding wheels were developed by Technodiamant. Further on, adequate dressing and profiling procedures have been set up in order to enable and assure the accurate machining of the moulds. The final specifications defined for the demonstrator forbid the use of a grinding procedure for machining the moulds due to the fact that the minimum radius of the grinding wheels was too large. To generate the blazed diffractive structures in an efficient way a cutting process by diamond turning was needed. The optimized tools by Contour withstand the process forces while machining and were precise enough to create blazed patterns at the same time.

To find a compatible mould material glass combination, extensive investigations and tests on machinability, mouldability, heating and glass contact, were made. In fact several moulds, aspherical and diffractive, were machined in relevant materials by conventional and ultrasonic-assisted diamond turning and were coated with different coatings. Afterwards the moulds were heated up with and without contact to the glasses defined by the demonstrators and were investigated by white light interferometery for example.

The mould manufacturing process offered some unexpected challenges. The optical design of the POG demonstrator allowed a mould manufacturing process without any big problems; a symmetrical diffractive structured mould on the one hand and an aspherical mould on the other hand. The roughness of the surface and the form accuracy of the aspherical mould offered typical values which can be achieved by ultra-precision diamond turning. The diffractive structure did not show significant defects. In contrast to that, the FocalSpec demonstrator consists of an aspherical side and a diffractive side. This diffractive structure is not symmetric to the centre point of the mould; it is an off-axis part of a larger structured area. Therefore two different concepts related to the moulding concepts were investigated. Based on the single cavity concept an off-axis mould with diffractive structure and one aspherical mould were manufactured. For the multi-cavity concept one large mould and two aspherical moulds were manufactured. It was not possible to machine the desired off-axis mould out of the chosen material. The tip of the tool broke due to the intermitted cut and the high forces while machining. After testing several parameters a PMMA positive part was machined and used for electroforming a Nickel-Cobalt-alloy negative. The PMMA material allowed to machine with a very small tool radius. The radius directly affects the efficiency of the diffractive structure. The electroformed negative part was coated and used for glass moulding and able to withstand the high temperatures during the moulding process. The machining of the mould for the multi-cavity concept could not be done with the very small tool. The tool could not withstand the high forces while structuring a quite large area. In fact the tool for a successful machining of the mould was 3 times bigger. The mould was coated and used for precision glass moulding, too. The aspherical mould offered similar values than the one from POG.

The main goal of the project was to enable the precision glass moulding process for the manufacturing of diffractive glass lenses. After the moulds were machined and coated successfully, the main challenge was to find practical parameters for a stable precision glass moulding process in order to guarantee a full moulding of the structure and the aspherical surface. For the FocalSpec and the off-axis diffractive structure PMMA-lenses were moulded to minimise the risk. Afterwards glass components were moulded. Unfortunately the needed sleeve broke while heating without any moulding force and the moulding of the FocalSpec demonstrator was not completed. But the structure was moulded into the glass partially and showed good measurement results. This leads to the interpretation that the moulding would be successful if the sleeve would be complete. The structure showed the expected characteristics. In fact a new sleeve will be machined and the glass demonstrators will be supplied to FocalSpec later. The PMMA-lenses worked very well in the measurement system. The measurement of the glass lenses of the multi-cavity concept did not show results as good as the one from the single-cavity concept due to the bigger tool radius while machining. But the moulding of the lenses was successful too. The challenge of the POG demonstrator was the moulding of the glass type. The first moulding tests lead to high forces in the lens and breaking of the glass. After several iteration steps a stable precision glass moulding process was developed.

The SME partners FocalSpec and POG can benefit from the developed process chain and the new glass lenses by applying them in new or revised systems. The additional information from the moulding and manufacturing process will help Aixtooling and Fraunhofer IPT for further projects and developments. The supplier of the manufacturing tools has successfully developed new or stable processes for the manufacturing of their tools.

Potential Impact:
The exploitation, dissemination and implementation of the projects outputs are essential elements of FP7 projects. In order to structure, schedule and document these activities a plan for use and dissemination of foreground was developed and updated regularly in course of the project. In order to make an easy file transfer a ftp-server was developed and regularly updated. A number of non-confidential results of the developed technology were published by the RTD performers. At the following conferences information and results have been presented::

- Oral presentation at the EOS Topical meeting on diffractive optics, Delft Netherlands (2012)
- Oral presentation at the EOSMOC, Munich Germany (2013)
- Oral presentation at the Optifab, Rochester USA (2013)

While the project period the following conferences and trade shows have been attented:

• Optics Colloquium 2012, 2014, (Germany)
• Euspen 2012, 2013 (Europe, yearly changing)
• EOS Conference 2012, 2013 (Germany)
• Photonics West 2012, 2013 (USA)
• Optifab 2011, 2013 (USA)
• Optatec 2012, 2014 (Germany)
• Laser 2013 (Germany)
• ODF 2012 (Japan)

List of Websites:
Below the involved partners of the project “NGOptics” are listed. For each partner just one involved person is listed.

RTD Partners:

Fraunhofer Institute for Production Technology IPT
Marius Doetz
Steinbachstr. 17
Germany, 52074 Aachen

Teknologian Tutkimuskeskus VTT
Jukka-Tapani Makinen
Kaitoväylä 1
Finnland, 90571 Oulu

SME Partners:

Aixtooling GmbH
Guido Pongs
Steinbachstr. 17
Germany, 52074 Aachen

Contour Fine Tooling BV
Eric van Hal
De Vest 1C
Netherlands, 5555 XL Valkenswaard

FocalSpec OY
Karri Niemela
Kaitoväylä 1
Finnland, 90571 Oulu

POG Präzisionsoptik Gera GmbH
Marc Schinkoeth
Gewerbepark Keplerstraße 35
Germany, 07549 Gera

Technodiamant Almere BV
Roland de Vries
Markerkant 13-11
Netherlands, 1314 Almere